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Tropical rainforest in South America

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According to a new study, tropical forests in South America lose their property as CO₂ reservoirs when it gets too hot. Due to the high temperatures, more trees are dying and the forest could even become an additional source of the climate-damaging gas. This is the result of studies by researchers at the University of Leeds, which were published in the journal Nature Climate Change.

The scientists analysed data from plots of the Amazon rainforest, the Atlantic forests and drier forests on the continent during the last El Niño event in 2015/2016. At that time, too, it was exceptionally hot in South America – similar to the currently prevailing El Niño.

Normally, forests act as so-called carbon sinks, removing more carbon from the air than they emit. They are therefore extremely important for mitigating the global climate crisis. "Tropical forests in the Amazon play a key role in slowing down the accumulation of climate-damaging carbon dioxide in the atmosphere," explains study co-author Amy Bennett from the University of Leeds.

Until now, scientists had known that trees in the Amazon rainforest, for example, are sensitive to changes in temperature and water availability. How sensitive individual forests are and how exactly they are changing due to higher temperatures in the climate crisis, however, is not. "Our results show what happened in the Amazon during the strong El Niño event and give us an idea of how much hot and dry weather affects forests." This would also make it possible to better assess future changes caused by the climate crisis.

The higher the temperature stress, the less CO₂ the forest stores

On average, an El Niño occurs every two to seven years. During the natural weather phenomenon, the ocean and air currents over the South Pacific change, which affects the world with extreme weather of various kinds and causes temperatures to rise overall.

Before the El Niño of 2015/2016, the forest plots would have sequestered about 0.3 tons of carbon per hectare per year. Under the hotter and drier conditions of the climate phenomenon, this value dropped to zero. According to the researchers, this was mainly due to the death of the trees. According to Beatriz Marimon of Mato Grosso State University in Brazil, some parts of the southeastern Amazon on the edge of the rainforest have gone from being a CO2 sink (which stores carbon) to a source of CO2. "While tree growth rates withstood the higher temperatures, tree mortality skyrocketed when El Niño occurred."

Temperatures on almost all plots rose by an average of 0.5 degrees Celsius, and most plots also suffered from water scarcity. According to the researchers, the already drier or damaged forests were particularly vulnerable.

The study is part of a larger research network on tropical forests. In total, more than 100 scientists are working on 123 experimental sites on the continent. Some scientists emphasized that in the entire study period of 30 years, the El Niño has so far not shown any worse effects on intact forests.

However, the phenomenon in 2015/2016 was stronger than ever before. For example, forests could die more quickly in the future if they are already weakened – for example due to deforestation – or if El Niño becomes even stronger in combination with global warming. Forests must be protected in such a way that they continue to serve as CO2 sinks and can survive extreme weather.